I have a single pipe heating system, and installed additional radiator which will not heat up. I have taken the supply from in the loft and come down to the ground floor, through radiator with bypass and tied back into the system on the ground floor. The radiator filled and was able to bleed all of the air out. It appears that there is no circulation, so maybe is being fed from both tie in points?. What can I do to make it work?

Are the radiator valves you fitted specifically designed for one pipe systems? How have you piped to the radiators? What size of pipe have you used. Has the pump the capacity to take an extra radiator?

The original piping is in 3/4", I have tied in with 15mm. The pump I believe is large enough as the old system worked set on 1 and I have change the setting to 2. The valves are just normal standard valves prchased with the radiator. Hope this helps.

I beleive that is where you problem lies, one pipe systems require very specific equipment to operate. i.e. low loss radiator valves.
The radiator valves should be large oriface valves or 3/4" as this will provide the low loss required. Search internet for one pipe radiator valves.
Hope this helps you.